1. Field of the Invention
The present invention relates to a vibration/ acceleration sensor for detecting the vibration of a substance and the acceleration caused due to such vibration by utilizing the electromechanical conversion characteristics of a piezoelectric material.
2. Statement of the Related Art
In general, conventional piezoelectric type vibration/acceleration sensors for detecting the elastic vibration of a vibrating body include those of a longitudinally effective type which utilize compression and tension in the thicknesswise direction of a piezoelectric element and those of a shear-effect type which utilize a shear force in the piezoelectric element. However, in the case where the resonance frequency is made to coincide with a specific vibration of a vibrating substance to detect only specified frequency components, or in the case where the vibration components in a specified region of frequencies are detected so as to increase the sensitivity in the region of low frequencies, a laterally effective type vibrator, i.e., a cantilever type vibrator utilizing a bending vibration mode has hitherto been widely employed. In case of the vibration/acceleration sensor having the cantilever type vibrator structure, a difficulty is encountered in satisfying conditions for fixing one end of the vibrator. In order to solve this problem, one conventional technique such as that disclosed in Japanese Patent Unexamined Publication No. 59-70923 stabilizes the fixing of one end of the vibrator with the use of a cantilever type bending vibrator constituting a vibration detecting portion which is fabricated by (1) forming a slit in a laminated plate-like member, such as a disc-like piezoelectric element, with the bending vibrator being integral, at one end, with the laminated plate-like piezoelectric element, and by (2) fixing the outer surface of the bending vibrator in place and supporting the same, thus making it possible to stabilize the fixing condition of the vibrator.
The above-described prior art piezoelectric vibration/acceleration sensor detects a mechanical force such as acceleration and, at the same time, generates an electrical charge in accordance with variations in the ambient temperature. This is due to the pyroelectric effect of a piezoelectric material and the electrical charge generation is given by the following formula: EQU dQ/dt=k dT/dt (1)
where Q represents the electrical charge, T the temperature, t the time, and k a proportional constant. Namely, the generation of this electrical charge can not be distinguished from the electrical charge produced due to an acceleration desired to be detected, causing a large error in detecting the degree of acceleration. The acceleration sensor of a structure wherein two sheets of piezoelectric materials are bonded together can theoretically achieve mutual cancellation of their generated charges in both types, that is, (1) a serial type in which the two sheets of piezoelectric material are bonded together so that the directions of the polarizing axes may become opposite to each other and so that the respective opposing surfaces of the two bonded sheets may serve as signal take-out electrodes, and (2) a parallel type in which the respective opposing surfaces of the two upper/lower bonded sheets are connected together to serve as a signal take-out electrode and the bonded surface electrode is also used as a signal take-out electrode. However, in the conventional acceleration sensor, transfer of heat to the piezoelectric material is non-uniform and in addition the electrical charges generated due to the pyroelectric effect fail to cancel each other out under severe temperature conditions so that output signals are generated from an asymmetrical electrode constituted by a bending vibration mode vibrator serving as the signal take-out portion. This makes it impossible to provide an acceleration sensor capable of achieving highly precise detection.